(1) Field of the Invention
The invention relates to a design of printed inverted-F antenna, and more particularly to a printed inverted-F antenna for communicating in dual frequency band and having a function of adjusting coupled impedance.
(2) Description of the Prior Art
Rapid innovation and development upon wireless communication technology have made mobile communication products as one of mainstream products nowadays. These mobile communication products include mobile phones, PDAs, notebooks, etc. They can couple with proper communication modules to link a Wireless Local Area Network (WLAN) for transmitting or/and receiving e-mail and instant information such as news, stocks quotations, and so on. In the art, the WLAN is an on-site wireless communication means that utilizes a WLAN card to transmit wirelessly vast data between computer systems. Apparently, in the WLAN, conventional complicated wiring webs have been replaced by wireless communication facilities. One of those wireless communication facilities is the antenna; in particular, a flat inverted-F antenna. The flat inverted-F antenna, characterized on its slim size and light weight, has been widely adopted as a built-in antenna in most of the mobile communication products.
Referring now to FIG. 1 for a conventional compact printed antenna, the antenna includes a substrate 10, a ground metal 12, a strip metal 20, a short circuit leg 14 and a feeding leg 16; in which the ground metal 12, the strip metal 20, the short circuit leg 14 and the feeding leg 16 are all printed circuits located on the substrate 10.
The ground metal 12 is shaped to form a coplanar wave guide (CPW) feeding structure 24 as shown in FIG. 1. The feeding leg 16 grows perpendicularly from the metal strip 20 and extends through the feeding structure 24 to further connect to a matching circuit (not shown in the drawing). The feeding leg 16 and the ground metal 12 are not connected with each other so as to avoid a short circuit problem. The strip metal 20 is parallel with the ground metal 12. The short circuit leg 14 is provided to bridge a short circuit end 18 of the strip metal 20 and the ground metal 12. On other hand, opposing to the short circuit end 18, an open circuit end 22 of the strip metal 20 is formed. The distance between the open circuit end 22 and the short circuit end 18 is preferably one quarter of a concerned wavelength. Alternatively in the art, one of another solutions of the inverted-F antenna is shown in FIG. 2, in which the ground metal 30 and the compact printed antenna including a conductive aperture 32, an open circuit end 34, a feeding leg 36, a metal strip 40, a short circuit end 42 are fabricated respectively on opposing surfaces of the substrate 38.
As the surface size of the compact printed antenna has a restriction that limits the length of the strip metal 20 to one quarter of the wavelength, the size of the antenna is thereby limited to a constant range of one quarter of the wavelength and thus cannot be shrunk effectively. Through the development of passive elements in the contemporary integrated circuits has been targeting at the miniaturization of elements, yet the antenna size of the communication products is still restricted by the unbreakable limitation of one-quarter signal wavelength
Besides, the operating frequency of the aforementioned compact printed antenna is limited to a single frequency band. For example, in a wireless local area network (WLAN), the operating frequency is usually located around ISM (Industrial Scientific Medical)2.4 GHz. Recently, noble wireless devices such as blue tooth apparatus are wildly adopted in wireless communication equipments. Hence, the interference problems such as co-channel interference and next-channel interference become much more serious. Also, it must be pointed out that the resonance frequency of the compact printed antenna between 8 GHz and 9 GHz is usually beyond the contemporary communication protocol. Therefore, the present invention is introduced not only to provide a shrunk size to the printed antenna but also to make the antenna operable under a dual-frequency band.